[ocaml-infra] Automatically synching GitHub pulls to OPAM

[Anil Madhavapeddy]

I've created a small script here that generates compiler entries for each GitHub PR.  This makes it exceedingly easy to play around with new pull requests.

https://github.com/avsm/opam-sync-github-prs

I can publish this regularly as a custom remote, but it occurs to me that during the 6-month GitHub experiment, we may as well merge this directly into OPAM stable.  Most users will not see all the extra repositories due to requiring the --all script.

Any thoughts, objections or improvements?  I decided not to include closed pull requests in the generation, but that can easily be tweaked.

-anil